This paper analyzed the cause of an insulation breakdown accident in the stator winding of a weather protected(WPII) high-voltage motor(1300kW, 6.6kV) that has been in operation for 27 years. As a result of the insulation diagnosis test conducted 1 year and 6 months before the accident, the polarization index, alternating current, and dielectric loss tangent test results were good, but the partial discharge test result was measured at 13,000pC and was evaluated as a condition requiring attention. In order to analyze the cause of the insulation breakdown of this motor, the damaged winding was cut and analyzed in detail, and no traces of insulation deterioration of the main insulator were found. Contamination was partially confirmed on the surface of the end winding, but it was also difficult to find traces of tracking. As a result of dismantling and analyzing the wires inside the winding, turn shorts were confirmed in two wires. Therefore, even if the high-voltage motor insulation diagnosis test result is judged to be good, it is necessary to comprehensively analyze the possibility of turn short circuit due to insulation deterioration inside of the main insulating material, tracking due to contamination, and deterioration of wire insulation.